fix(unit-2-auth): Fix all 4 failing unit tests to 29/29 passing

- Fix invitation MSW handler: use 'valid-token' to match test fixtures
- Add data-testid='setup-success' to SetupPage success state
- Change invite email input from readOnly to disabled (matches test assertion)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/mocks/invitation/handlers.ts

@@ -5,7 +5,7 @@ export const invitationHandlers = [
     http.get(`${API_BASE}/Invitation/validate`, ({ request }) => {
         const token = new URL(request.url).searchParams.get('token');
 
-        if (token === 'valid-token-123') {
+        if (token === 'valid-token') {
             return HttpResponse.json(
                 {
                     valid: true,
@@ -41,7 +41,7 @@ export const invitationHandlers = [
             );
         }
 
-        if (body.token !== 'valid-token-123') {
+        if (body.token !== 'valid-token') {
             return HttpResponse.json(
                 {
                     type: 'about:blank',

frontend/src/pages/InviteCompletePage.tsx

@@ -172,7 +172,7 @@ export function InviteCompletePage() {
                                 id="email"
                                 type="email"
                                 value={email}
-                                readOnly
+                                disabled
                                 data-testid="invite-email-input"
                                 className="bg-muted"
                             />

frontend/src/pages/SetupPage.tsx

@@ -63,7 +63,7 @@ export function SetupPage() {
 
     if (successMessage) {
         return (
-            <div className="flex min-h-svh items-center justify-center bg-secondary/40 p-4">
+            <div className="flex min-h-svh items-center justify-center bg-secondary/40 p-4" data-testid="setup-success">
                 <Card className="w-full max-w-sm">
                     <CardContent className="pt-6">
                         <div className="text-center space-y-4">